WO2009115435A1

WO2009115435A1 - Submicronic barium and magnesium aluminate, method for making same and use thereof as a phosphor

Info

Publication number: WO2009115435A1
Application number: PCT/EP2009/052795
Authority: WO
Inventors: Valérie BUISSETTE; Thierry Le-Mercier; Laurent Thiers; Yvan Montardi; Olivier Le-Roux
Original assignee: Rhodia Operations
Priority date: 2008-03-18
Filing date: 2009-03-10
Publication date: 2009-09-24
Also published as: FR2928912A1; JP2011517466A; CN105694881A; US20110068303A1; JP5356497B2; EP2265690B1; KR20100119891A; KR101316955B1; KR20130020926A; FR2928912B1; CA2716595C; CN101978022A; ES2745953T3; US8580150B2; EP2265690A1; CA2716595A1

Abstract

The barium and magnesium aluminate of the present invention is characterised in that the same is in the form of a liquid-phase suspension of substantially monocrystalline particles having an average size of between 80 and 400 nm. The aluminate is prepared by a method that comprises: forming a liquid mixture containing compounds of aluminium and of other elements that are part of the aluminate composition; drying the mixture by atomisation; calcining the dried product in a reducing atmosphere and wet-grinding said product. The aluminate of the invention can be used as a phosphor.

Description

BARIUM ALUMINATE AND SUBMICRONIC MAGNESIUM, PROCESS FOR THEIR PREPARATION AND USE AS A LUMINOPHORE

The present invention relates to a submicron barium and magnesium aluminate, its method of preparation and the use of this aluminate as a phosphor.

The fields of luminescence and electronics are currently undergoing important developments. An example of these developments is the development of plasma systems (screens and lamps) for new visualization, lighting or marking techniques. These new applications require phosphor materials with further improved properties. Thus, in addition to their luminescence property, these materials are required to have specific characteristics of morphology or grain size in order, in particular, to facilitate their implementation in the desired applications.

More specifically, it is required to have luminophores in the form of particles as possible as individualized and very thin, submicron, especially less than 500 nm. Methods for the preparation of phosphors by champing are known. However, in order to obtain the desired crystallographic phases, these processes require calcination at a high temperature, which generally results in products that are difficult to grind so that it is impossible to achieve such a size. Moreover and always in the development in the fields of luminescence and electronics, we seek to obtain materials in the form of thin layers, transparent and luminescent.

The main object of the invention is to provide products having such granulometric characteristics. A second object of the invention is to obtain a luminescent material of the above type.

For this purpose, the barium magnesium aluminate of the invention is characterized in that it is in the form of a suspension in a liquid phase of substantially monocrystalline particles of average size between 80 and 400 nm.

Other characteristics, details and advantages of the invention will appear even more completely on reading the description which follows, as well as the appended drawings in which: FIG. 1 is an RX diagram of an aluminate according to the invention;

FIG. 2 is an emission spectrum of this same aluminate;

FIG. 3 is a TEM photo of a second suspension according to the invention; FIG. 4 is a TEM photo of a third suspension according to the invention;

FIG. 5 is an emission spectrum of a fourth aluminate according to the invention;

FIG. 6 is an emission spectrum of a fifth aluminate according to the invention.

In the present description, rare earth is understood to mean the elements of the group consisting of yttrium and the elements of the Periodic Table with an atomic number inclusive of between 57 and 71.

The aluminate of the invention consists of particles which have the essential characteristic of being submicron and monocrystalline.

More specifically, these particles have an average size (dso) of between 80 and 400 nm, more particularly between 100 and 300 nm. This size can be between 80 and 200 nm and even more particularly between 100 and 200 nm. For certain applications of the aluminate of the invention, for example for the manufacture of a transparent material, as described below, it is possible to use an aluminate whose particles have a size of between 100 nm and 150 nm.

Moreover, these particles may have a narrow particle size dispersion; more precisely their dispersion index may be at most 1, preferably at most 0.7 and even more preferably at most 0.5.

For the whole of the description, the average size and the dispersion index are the values obtained by implementing the laser diffraction technique using a laser granulometer (volume distribution). The dispersion index is the ratio:

in which :

d ₈₄ is the particle diameter for which 84% of the particles have a diameter of less than ₈₄ ; - die is the particle diameter for which 16% of the particles have a diameter less than d-iβ;

d ₅₀ is the average diameter of the particles. It is specified here that the measurements of average size are made on suspensions which have not undergone decantation, that is to say without phase supernatant and no sedimentation pellet, and which, if necessary, have been treated by a transition to the ultrasound probe according to well known methods implemented for this type of measurements.

The other characteristic of the constitutive particles of the aluminate of the invention is their monochstallinity. Indeed, for the most part, that is to say for at least 90% of them and, preferably for all of them, these particles consist of a single crystal. This monocrystalline aspect of the particles can be demonstrated by the transmission electron microscopy (TEM) analysis technique.

For suspensions whose particles are in a size range of at most about 200 nm, the monocrystalline appearance of the particles can also be demonstrated by comparing the average particle size measured by the laser diffraction technique mentioned above with the value of the measurement of crystal size or coherent domain obtained from X-ray diffraction analysis (XRD). It is specified here that the value measured in XRD corresponds to the size of the coherent domain calculated from the diffraction line corresponding to the crystallographic plane [102]. The two values: average laser diffraction size and XRD in fact have the same order of magnitude, ie they are in a ratio (measured value dso / measured value XRD) less than 2, more particularly at most 1, 5.

As a consequence of their monocrystalline character, the particles of the aluminate of the invention are in a well separated and individualized form. There are no or few agglomerates of particles. This good individualization of the particles can be demonstrated by comparing the dso measured by the laser diffraction technique with that measured from an image obtained by transmission electron microscopy (TEM). Thus, for a given value of the d50 measured by the laser diffraction technique (dso laser value), the value measured by MET (MET value) is at least equal to (dso value laser) / 2 and the ratio value dso laser / value MET can be between 1 and 2.

The aluminate of the invention is based on aluminum, barium and magnesium oxide but may contain additional elements, called "substituents" or "dopants" because these elements are considered to be a partial substitution constituent elements Ba, Mg and Al and they allow in particular to modify the optical and luminescence properties of the aluminate.

Examples of these substituents will be given below for each constituent element on the basis of what is currently accepted in the state of the art. This implies that it would not depart from the present invention if a substitute described for a given constituent element would in fact be in fact substituted for another constituent element than that presumed in the present description.

Thus, the barium may be partially substituted by at least one rare earth which may be in particular gadolinium, terbium, yttrium, ytterbium, europium, neodymium and dysprosium, these elements may be taken alone or in combination. Similarly, the magnesium may be partially substituted by at least one element selected from zinc, manganese or cobalt.

Finally, the aluminum may also be partially substituted by at least one element selected from gallium, scandium, boron, germanium or silicon.

The amounts of these substituents may vary, in known manner, in wide ranges, however they must be such, for maximum values, that the crystallographic structure of the aluminate is substantially preserved. On the other hand, the minimum amount of substituent is that below which the substituent no longer produces an effect. Generally, however, the amount of barium substituent is at most 40%, more particularly at most 20% and even more particularly at most 10%, this amount being expressed as atomic% (atomic ratio substituent / (substituent For magnesium, this quantity, expressed in the same manner, is generally at most 60%, more particularly at most 40% and even more particularly at most 10%. this quantity, always expressed in the same way, is generally at most 15%, for example the minimum amount of substituent may be at least 0.1%.

For example, the aluminate of the invention may have the formula (I) below: a (Bai-dM ^_1d O) .b (Mg _1- eM ² eO) .c (AI ₂ θ3 ) (I) wherein: M ¹ denotes a rare earth which may be more particularly gadolinium, terbium, yttrium, ytterbium, europium, neodymium and dysprosium;

M ² denotes zinc, manganese or cobalt; a, b, c, d and e check relationships: 0.25 ≤ a ≤ 2; 0 <b ≤ 2; 3 ≤ c ≤ 9; 0 ≤ d ≤ 0.4 and 0 ≤ e ≤ 0.6.

M ¹ can be even more particularly the europium.

M ² may be more particularly manganese.

More particularly, the aluminate of the invention can meet the formula (I) above wherein a = b = 1 and c = 5.

According to another particular embodiment, the aluminate of the invention can meet the formula (I) above in which a = b = 1 and c = 7.

According to yet another particular embodiment, the aluminate of the invention may meet the formula (I) above in which a = 1; b = 2 and c = 8.

Examples of this type of product are those of formula BaMgAl ₂ OO ₇ ; Ba ₀ , 9Eu ₀ , iMgAlioOi ₇ ; Bao, 9Euo, 1Mgo, 6Mn ₀ , ₄ Al10O ₇ ; Bao, 9Euo, 1Mgo, 8Mn ₀ , ₂ Al10O ₇ ;

; BaMgAI - | ₄ O ₂ 3; Ba ₀ , 9Euo, iMgAlI ₄ O ₂ 3; Ba _0, 8Euo, Mgi ₂ 9 ₃ Mno, o7Ali6θ _27. The aluminate of the invention is generally in the form of a suspension in a liquid phase of the particles which have just been described.

With regard to the solid particles, these consist essentially or solely of aluminate as described above, they contain no other compounds than this aluminate, except for example any impurities in very small quantity. The aluminate is crystallized in the form essentially of a beta alumina. This crystallization is evidenced by X-ray analysis. By "essentially" it is meant that the X-ray diagram obtained by the analysis made on the dried powder resulting from the suspension of the invention may have, in addition to the majority phase of beta-alumina, one or more minority phases corresponding to impurities such as that, for example, BaAI ₂ O ₄ . According to a preferred embodiment of the invention, the aluminate is crystallized in the form of a pure phase of beta alumina. In this case, the XRD analysis reveals only one crystallographic phase. This suspension is stable, that is to say that there is no sedimentation of the solid particles for several hours, for example over a period of about 24 hours.

However, over time, sedimentation can occur which can cause the particles to agglomerate with each other. However, and this is an important property of the suspension of the invention, a simple agitation using a very low mechanical energy, in particular an ultrasonic treatment, for example with a power of 120 W for 3 minutes, makes it possible to disaggregate these particles and therefore to come back to a suspension whose particles have all the characteristics that have been given above.

The liquid phase of the suspensions according to the invention may be water or a mixture of water / solvent miscible with water or an organic solvent. The organic solvent may be very particularly a solvent miscible with water. There may be mentioned, for example, alcohols such as methanol or ethanol, glycols such as ethylene glycol, acetate derivatives of glycols such as ethylene glycol monoacetate, glycol ethers, polyols or ketones. This liquid phase may also include a dispersant.

This dispersant can be chosen from known dispersants, for example from polyphosphates (IvWPnOs _{n +} -I) or metaphosphates ([MPOs] _n ) which are alkaline (M denotes an alkaline such as sodium), especially as sodium hexametaphosphate. It can also be chosen from alkali silicates (sodium silicate), amino alcohols, phosphonates, citric acid and its salts, phosphosuccinic acid derivatives ((HOOC) _n -R-PO ₃ H ₂ where R is an alkyl chain), polyacrylic, polymethacrylic, polystyrene sulfonic acids and their salts. Citric acid and metaphosphates are particularly preferred. The amount of dispersant may be between 1% and 15%, more particularly between 4% and 8%, this amount being expressed as mass of dispersant relative to the mass of solid in the dispersion.

The concentration of the suspension can vary over a wide range. For example, it may be between about 10 g / l and about 500 g / l, more particularly between 40 g / l and 300 g / l, this concentration being expressed as mass of solid per volume of suspension.

For suspensions in aqueous phase, and according to a particular variant, an interesting feature of these is their stability in a high pH range. Thus, these suspensions remain stable, that is to say here they do not give rise to sedimentation and agglomeration of the particles when their pH is caused to vary within given ranges, all of these ranges generally covering pH values which may be between 2 and 11, more particularly between 4 and 10. Such variations in pH may occur in the implementation of suspensions in luminescence applications and this characteristic is therefore important because the suspensions can be used in a wide range of applications. The invention also relates to an aluminate which is in solid form, that is to say a powder which has the characteristic of being able to lead to the aluminate in suspension form described above. In other words, when this powder is redispersed in a liquid phase, after simple stirring, without the need to apply a large mechanical energy, including, again, by simple ultrasonic treatment, for example with a approximately 450W power, a suspension of the aluminate having the characteristics given above is obtained.

The process for preparing the aluminate of the invention in suspension form will now be described.

This method comprises a first step in which a liquid mixture is formed which is a solution or a suspension or a gel, aluminum compounds and other elements included in the composition of the aluminate. As compounds of these elements, inorganic salts or hydroxides or carbonates are usually used. As salts, mention may be made of nitrates preferably, in particular for barium, aluminum, europium and magnesium. Sulfates, especially for aluminum, chlorides or organic salts, for example acetates, may optionally be employed.

It is also possible to use as aluminum compound a sol or a colloidal dispersion of aluminum. Such a colloidal dispersion of aluminum may have particles or colloids whose size is between 1 nm and 300 nm. Aluminum can be present in the soil as boehmite. The next step is to dry the previously prepared mixture.

This drying is done by atomization.

Spray drying is understood to mean spray drying of the mixture in a hot atmosphere (spray-drying). The atomization can be carried out using any sprayer known per se, for example by a spraying nozzle of the watering apple or other type. It is also possible to use so-called turbine atomizers. On the various spraying techniques that can be implemented in the present process, reference may be made in particular to the basic work of MASTERS entitled "SPRAY-DRYING" (second edition, 1976, Editions George Godwin - London).

It will be noted that the atomization-drying operation can also be implemented by means of a "flash" reactor, for example of the type described in French Patent Application Nos. 2 257 326, 2 419 754 and 2,431 321. This type of atomizer can be used in particular to prepare products whose particle size is low. In this case, the treating gases (hot gases) are driven by a helical movement and flow into a vortex well. The mixture to be dried is injected along a path coinciding with the axis of symmetry of the helical trajectories of said gases, which makes it possible to perfectly transfer the momentum of the gases to the mixture to be treated. The gases thus provide a dual function: on the one hand the spraying, ie the transformation into fine droplets, of the initial mixture, and on the other hand the drying of the droplets obtained. Furthermore, the extremely low residence time (generally less than 1/10 of a second) of the particles in the reactor has the advantage, among other things, of limiting possible risks of overheating due to too long contact with the particles. hot gases.

With regard to the flash reactor mentioned above, reference may in particular be made to FIG. 1 of the French patent application 2,431,321.

It consists of a combustion chamber and a contact chamber consisting of a bicone or a truncated cone whose upper part diverges. The combustion chamber opens into the contact chamber through a reduced passage.

The upper part of the combustion chamber is provided with an opening allowing the introduction of the fuel phase.

On the other hand, the combustion chamber comprises a coaxial inner cylinder, thus defining inside it a central zone and an annular peripheral zone, with perforations situated for the most part towards the upper part of the apparatus. The chamber comprises at least six perforations distributed over at least one circle, but preferably on several circles spaced axially. The total area of the perforations located in the lower part of the chamber may be very small, of the order of 1/10 to 1/100 of the total surface area of the perforations of said coaxial inner cylinder.

The perforations are usually circular and have a very small thickness. Preferably, the ratio of the diameter thereof to the thickness of the wall is at least 5, the minimum thickness of the wall being limited only by the mechanical requirements.

Finally, an angled pipe opens into the reduced passage, the end of which opens in the axis of the central zone. The gas phase animated by a helical movement (hereinafter called helicoidal phase) is composed of a gas, generally air, introduced into an orifice made in the annular zone, preferably this orifice is situated in the lower part of said zone. In order to obtain a helicoidal phase at the reduced passage, the gaseous phase is preferably introduced at low pressure into the aforementioned orifice, that is to say at a pressure of less than 1 bar and more particularly at a pressure comprised between between 0.2 and 0.5 bar above the pressure in the contact chamber. The speed of this helicoidal phase is generally between 10 and 100 m / s and preferably between 30 and 60 m / s.

Furthermore, a fuel phase, which may in particular be methane, is injected axially through the above-mentioned opening in the central zone at a speed of approximately 100 to 150 m / s. The fuel phase is ignited by any known means, in the region where the fuel and the helical phase are in contact.

Subsequently, the imposed passage of gases in the reduced passage is made according to a set of trajectories confused with families of generators of a hyperboloid. These generators are based on a family of circles, small rings located near and below the reduced passage, before diverging in all directions.

The mixture to be treated is then introduced in liquid form through the aforementioned pipe. The liquid is then fractionated into a multitude of drops, each of which is transported by a volume of gas and subjected to a movement creating a centrifugal effect. Usually, the flow rate of the liquid is between 0.03 and 10 m / s.

The ratio of the intrinsic momentum of the helical phase to that of the liquid mixture must be high. In particular, it is at least 100 and preferably between 1000 and 10000. The amounts of movement at the reduced passage are calculated as a function of the inlet flow rates of the gas and of the mixture to be treated, as well as the section of the passage. An increase in flow leads to a growth in the size of the drops.

Under these conditions, the proper movement of the gases is imposed in its direction and its intensity to the drops of the mixture to be treated, separated from each other in the convergence zone of the two currents. The speed of the liquid mixture is further reduced to the minimum necessary to obtain a continuous flow. Atomization is generally carried out with a solids outlet temperature of between 100 ^° C. and 300 ^° C.

The next step of the process is to calcine the product obtained after drying. This calcination is at a temperature which is sufficiently high to obtain a crystalline phase. Generally, this temperature is at least 1100 ° C., more particularly at least 1200 ^° C. It may be at most 1500 ^° C. and for example be between 1200 ° C. and 1400 ° C.

This calcination is carried out under air or, in particular, in the case where the aluminate contains a dopant and for uses of this aluminate as a phosphor, under a reducing atmosphere, for example under hydrogen mixed in nitrogen or argon. The duration of this calcination is for example between 30 minutes and about 10 hours. It is possible to make two calcinations, the first under air and the second under a reducing atmosphere. In some cases, depending on the type of starting compounds selected from aluminum and other elements, it may be advantageous to make a preliminary calcination to that or those which has or have been mentioned above, at a temperature above slightly lower than those given above, for example below 1000 ^° C. The last step of the process consists in grinding the product resulting from the calcination. According to the invention, wet grinding is carried out in water or in a water / solvent mixture or in an organic solvent of the same type as the solvents which have been described above for the liquid phase constituting the suspension. A dispersant of the type of those described above and in the amounts given above can be used during grinding. This dispersant can contribute to the stability of the suspension obtained in different pH ranges as described above, a given dispersant causing stability in a given pH range. The wet grinding is done under conditions that are otherwise well known to those skilled in the art.

At the end of the wet grinding, the aluminate of the invention is obtained in the form of a suspension.

It will be noted that in the case of a suspension in a water / solvent mixture or in an organic solvent, this suspension can be prepared from an aqueous suspension as obtained by the process just described and by addition. organic solvent to this aqueous suspension and, if necessary, distillation to remove water. The description that has just been made relates to the preparation of aluminate in the form of a suspension. In order to obtain the aluminate of the invention in the form of a powder, this suspension is used and the solid product is separated from the liquid phase using any known separation technique, for example by filtration. The solid product thus obtained may be dried optionally and then resuspended in a liquid phase of the same type as that described above.

Due to their properties and the nature of the substituents or dopants, the aluminates of the invention are understood to mean the aluminates in the form of a suspension or the aluminates in solid form, can be used as luminophores.

More precisely, these aluminates exhibit luminescence properties under electromagnetic excitation in the wavelength range used in plasma systems (screens and lamps where the excitation is created by a rare gas or a rare gas mixture such as xenon and / or neon), mercury vapor lamps and light-emitting diodes (LEDs). As a result, they can be used as luminophores in plasma systems (display or lighting system), mercury vapor lamps and LEDs. The invention therefore also relates to luminescent devices comprising the aluminate described above or as obtained by the method described above or manufactured using this same aluminate. Similarly, the invention relates to plasma systems, mercury vapor lamps or LEDs, in the manufacture of which the aluminate can enter, or comprising the same aluminate. The implementation of phosphors in these fabrications is done according to well-known techniques for example by screen printing, electrophoresis, sedimentation, inkjet, spraying, "spin-coating" or "dip-coating".

The particle size properties of the aluminates of the invention make them suitable for use as markers in semitransparent inks, for example for marking with an invisible bar code system.

The aluminates of the invention can also be used as markers in a material such as paper, cardboard, textile, glass or a macromolecular material. This can be of different types: elastomeric, thermoplastic, thermosetting.

On the other hand, the particular properties of these aluminates, when they are not doped, (no absorption in the visible range and UV), make them can be used as a reflective barrier in mercury vapor lighting systems.

The invention also relates to a luminescent material which comprises, or may be manufactured using, at least one aluminate according to the invention or an aluminate obtained by the process as described above.

According to a preferred embodiment, this luminescent material may be furthermore transparent. In this case, the aluminate entering into its composition or in its manufacture is an aluminate according to the invention and of average size between 100 nm and 200 nm, preferably between 100 nm and 150 nm.

It will be noted that this material may comprise, or be manufactured using, besides the aluminate of the invention, other aluminates, or more generally, other luminophores, in the form of submicron or nanometric particles. This material can be in two forms, that is to say either in a mass form, the whole of the material having the properties of transparency and luminescence is in a composite form, that is to say in this case in the form of a substrate and a layer on this substrate, the layer then only having these properties of transparency and luminescence. In this case, the aluminate of the invention is contained in said layer.

The substrate of the material is a substrate which may be silicon, silicone-based or quartz-based. It can also be a glass or a polymer such as polycarbonate. The substrate, for example the polymer, may be in a rigid form or in a flexible form such as a sheet or a plate a few millimeters thick. It can also be in the form of a film of a few tens of microns or even a few microns to a few tenths of a millimeter thick.

For the purpose of the invention, the term "transparent material" means a material which has a haze of at most 60% and a total transmission of at least 60% and preferably a haze of at most 40% and a total transmission of at least 80%. The total transmission is the amount of total light that passes through the layer, relative to the amount of incident light. The haze corresponds to the ratio of the diffuse transmission of the layer to its total transmission.

These two quantities are measured under the following conditions: the layer of material with a thickness of between 0.2 μm and 1 μm is deposited on a standard glass substrate, 0.5 mm thick. Mass fraction in aluminate particles in the material is at least 20%. The total transmission and diffuse transmission measurements are made through the material and substrate layer, using a standard procedure on a Perkin Elmer Lamda 900 spectrometer, equipped with an integrating sphere, for a wavelength of 550 nm.

The material, and more particularly the aforementioned layer, may comprise, besides an aluminate according to the invention, binders or fillers of the polymer (polycarbonate, methacrylate), silicate, silica beads, phosphate, titanium oxide or other mineral fillers type. to improve in particular the mechanical and optical properties of the material.

The mass fraction of aluminate particles in the material may be between 20% and 99%.

The thickness of the layer may be between 30 nm and 10 μm, preferably between 100 nm and 3 μm and even more preferably between 100 nm and 1 μm.

The material, in its composite form, can be obtained by depositing on the substrate, optionally previously washed for example with a sulfo-chromic mixture, an aluminate suspension of the invention. It is also possible to add at the time of this deposit, binders or charges mentioned above. This deposit can be achieved by a spraying technique, "spin-coating" or "dip-coating". After deposition of the layer, the substrate is dried in air and it can optionally subsequently undergo a heat treatment. The heat treatment is carried out by heating at a temperature which is generally at least 200 ^° C. and the higher value of which is fixed in particular taking into account the compatibility of the layer with the substrate so as to avoid interfering reactions in particular. The drying and the heat treatment can be conducted under air, under an inert atmosphere, under vacuum or under hydrogen.

It has been seen above that the material may comprise binders or fillers. It is possible in this case to use suspensions which themselves comprise at least one of these binders or these fillers or precursors thereof.

The material in the mass form can be obtained by incorporating the aluminate particles in a polymer type matrix for example, such as polycarbonate, polymethacrylate or silicone.

The invention finally relates to a luminescent system which comprises a material of the type described above and, in addition, an excitation source which can be a source of UV photons, such as a UV diode or an excitation type Hg, rare gases or X-rays.

The system can be used as a transparent wall lighting device, of the illuminating glazing type. Examples will now be given.

EXAMPLE 1

This example relates to the preparation of a suspension of a barium aluminate and magnesium of formula Bao _{, 9} Euo _, iMgAli ₀ Oi ₇ , according to the invention. A solution consists of a mixture of nitrates of barium, magnesium and europium, of the following composition (in atomic%):

Ba: 45%

Mg: 50%

Eu: 5% In addition, a boehmite sol (specific surface 265 m ² / g) is produced, with an Al concentration of about 1.8 mole / l. The nitrate solution and the boehmite sol are mixed to obtain a gel having the following molar ratios:

Ba / Al: 0.09 Mg / Al: 0.1

Eu / Ai: 0.01

Water is added to this gel to obtain an Al concentration of about 0.7 mol / l. The gel thus obtained has a final pH of 3.5. The gel is dried on a flash type atomizer as described above and in FR 2431321 A1, with a temperature of 180 ⁰ C output. The dried powder is then calcined under air at 900 ^° C. for 2 hours, then under an Ar / H ₂ mixture (95/5) at 1400 ° C. for 2 hours.

The powder obtained is subjected to wet grinding in a Netzch Labstar ball mill, with 0.4-0.8 mm ZrO ₂ -SiO ₂ balls. The degree of occupation of the balls in the grinding chamber is 70%. The concentration of the suspension is 20% by mass of solid and a dispersant, sodium hexametaphosphate (HMP), is added at a rate of

0.025 g of HMP / g powder (ie 2.5% by weight). The mill is used in recirculation, with a rotation speed of 3000 rpm. The grinding lasts 90 minutes.

The analysis by laser granulometry, without ultrasound, of the suspension obtained gives the following results:

As appears from FIG. 1, analysis of the sample obtained by drying the suspension at 100 ^° C. in an oven by X-ray diffraction reveals a beta-alumina phase, with a coherent domain size calculated from the diffraction line corresponding to the crystallographic plane [102] of 101 nm.

It is noted that the value of the d ₅ o (laser) and that of the size of the coherent domain (DRX) have the same order of magnitude, which confirms the monocrystalline character of the particles.

The suspension obtained emits in blue (450 nm) under excitation at 254 nm. Figure 2 is the emission spectrum of this suspension.

EXAMPLE 2

This example relates to the preparation of a suspension of a barium aluminate and magnesium of formula Bao _{, 9} Euo _, iMgAli ₀ Oi ₇ , according to the invention.

The preparation is identical to that of Example 1, until calcination at 1400 ^° C.

The powder obtained is subjected to wet grinding in a Molinex ball mill, with 0.4-0.6 mm ZrO 2 -SiO 2 beads. The degree of occupation of the balls in the grinding chamber is 65%. The concentration of the suspension is 20% by weight of solid and a dispersant, sodium citrate, is added at a level of 0.05 g of Na citrate / g powder (ie 5% by mass). The rotational speed of the mobile is 1000 rpm. The grinding lasts 95 minutes.

Analysis of the sample obtained by drying the suspension at 100 ° C. in an oven by X-ray diffraction reveals a beta-alumina phase, with a coherent domain size calculated from the diffraction line corresponding to the crystallographic plane [102] of 1 19 nm.

It is noted that the value of the d ₅ o (laser) and the size of the coherent domain have the same order of magnitude, which confirms the monocrystalline nature of the particles. Moreover, FIG. 3 is a TEM picture of the suspension resulting from grinding. This photo shows the monocrystalline character of the particles.

The suspension obtained emits in blue (450 nm) under excitation at 254 nm.

EXAMPLE 3

The powder obtained is subjected to wet grinding in a Molinex ball mill, with 0.4-0.6 mm ZrO 2 -SiO 2 beads. The degree of occupation of the balls in the grinding chamber is 65%. The concentration of the suspension is 20% by weight of solid and a dispersant, phosphosuccinic acid, is added at a level of 0.09 g of phosphosuccinic acid / g powder (ie 9% by mass). The rotational speed of the mobile is 1000 rpm. The grinding lasts 150 minutes.

FIG. 4 is a TEM photo of the suspension resulting from grinding, which shows the monocrystalline character of the particles.

The suspension obtained emits in blue (450 nm) under excitation at 254 nm.

EXAMPLE 4

The procedure is as in Example 1 until a gel having a final pH of 3.5 is obtained. The gel is dried on a type atomizer APV ^® , with a temperature of 145 ° C output. The dried powder is then calcined in air at 900 ^° C. for 2 hours, then under a mixture of Ar / H ₂ (95/5) at 1400 ^° C. for 2 hours.

The resulting powder is wet milled in a Molinex ball mill, with 1.6-2.5 mm ZrO ₂ -SiO ₂ beads. The degree of occupation of the balls in the grinding chamber is 65%. The concentration of the suspension is 50% by mass of solid and a dispersant, sodium hexametaphosphate (HMP) is added at a level of 0.05 g of HMP / g powder (ie 5% by mass). The rotational speed of the mobile is 1800 rpm. The grinding lasts 240 minutes.

Following this wet grinding, a second grinding is performed on the suspension with 0.2-0.3 mm ZrO ₂ -SiO ₂ balls for 45 minutes, without modifying the other grinding parameters.

The analysis of the sample obtained by drying the suspension at 100 ° C. in an oven by X-ray diffraction reveals a beta-alumina phase, with a coherent domain size calculated from the diffraction line corresponding to the crystallographic plane. [102] of 100 nm.

It is noted that the value of the dso (laser) and that of the size of the coherent domain have the same order of magnitude, which confirms the monocrystalline nature of the particles. The suspension obtained emits in blue (450 nm) under excitation at 254 nm.

EXAMPLE 5

This example relates to the preparation of a suspension of a barium and magnesium aluminate of formula Ba 0.89 Mo 0.95Mn 0.05 Al ₁₀ _O 17, according to the invention.

One solution consists of a mixture of nitrates of barium, magnesium, europium and manganese, of the following composition (in atomic%): Ba: 45%

Mg: 47.5%

Eu: 5%

Mn: 2.5% In addition, a boehmite sol (specific surface 265 m ² / g) is produced, with an Al concentration of about 1.8 mole / l. The nitrate solution and the boehmite sol are mixed to obtain a gel having the following molar ratios:

Ba / Al: 0.09 Mg / Al: 0.095

Eu / Ai: 0.01

Mn / AI: 0.005

Water is added to this gel to obtain an Al concentration of about 0.7 mol / l. The gel thus obtained has a final pH of 3.5. The gel is dried on equipment identical to that of Example 1 with a temperature of 180 ⁰ C output. The dried powder is then calcined under air at 900 ^° C. for 2 hours, then under an Ar / H ₂ mixture (95/5) at 1400 ° C. for 2 hours.

The resulting powder is wet milled in a Molinex ball mill, with 1.6-2.5 mm ZrO ₂ -SiO ₂ beads. The degree of occupation of the balls in the grinding chamber is 65%. The concentration of the suspension is 50% by mass of solid and a dispersant, sodium hexametaphosphate (HMP) is added at a rate of

0.075 g of HMP / g powder (ie 7.5% by mass). The rotational speed of the mobile is 1800 rpm. The grinding lasts 360 minutes.

Following this wet grinding, a second grinding is performed on the suspension with 0.2-0.3 mm ZrO ₂ -SiO ₂ balls. The other grinding parameters do not change, and the grinding time is 215 minutes. The analysis by laser granulometry, without ultrasound, of the suspension obtained gives the following results:

Analysis of the sample obtained by drying the suspension at 100 ^° C. in an oven by X-ray diffraction reveals a beta-alumina phase, with a coherent domain size calculated from the diffraction line corresponding to the crystallographic plane. [102] of 10 nm. It is noted that the value of the d ₅ o (laser) and the size of the coherent domain have the same order of magnitude, which confirms the monocrystalline nature of the particles.

The product obtained emits in blue (450 nm) and green (515 nm) under excitation at 254 nm. Figure 5 is the emission spectrum of this suspension.

EXAMPLE 6

This example relates to the preparation of a suspension of a barium aluminate and magnesium of formula Bao, 9Euo, iMgo, 6Mn ₀ , ₄ AlioOi7, according to the invention. One solution consists of a mixture of nitrates of barium, magnesium, europium and manganese, of the following composition (in atomic%):

Ba: 45%

Mg: 30% Eu: 5%

Mn: 20%

On the other hand, a boehmite sol (specific surface of 265 m ² / g) is manufactured with an Al concentration of about 1.8 mole / l. The nitrate solution and the boehmite sol are mixed to obtain a gel having the following molar ratios:

Ba / Al: 0.09

Mg / Al: 0.06

Eu / Ai: 0.01

Mn / Al: 0.04 Water is added to this gel, to obtain an Al concentration of about 0.7 mol / l. The gel thus obtained has a final pH of 3.5. The gel is dried on equipment identical to that of Example 1 with a temperature of 180 ⁰ C output. The dried powder is then calcined in air at 900 ° C. for 2 hours and then under a mixture of Ar / H ₂ (95/5) at 1400 ^° C. for 2 hours.

The resulting powder is wet milled in a Molinex ball mill, with 1.6-2.5 mm ZrO ₂ -SiO ₂ beads. The degree of occupation of the balls in the grinding chamber is 65%. The concentration of the suspension is 50% by weight of solid and a dispersant, sodium hexametaphosphate (HMP) is added at a rate of 0.075 g of HMP / g powder (or 7.5% by mass). The rotational speed of the mobile is 1800 rpm. The grinding lasts 420 minutes.

Following this wet grinding, a second grinding is performed on the suspension with 0.2-0.3 mm ZrO 2 -SiO 2 beads. The other grinding parameters do not change and grinding time is 200 minutes.

Analysis of the sample obtained by drying the suspension at 100 ^° C. in an oven by X-ray diffraction reveals a beta-alumina phase, with a coherent domain size calculated from the diffraction line corresponding to the crystallographic plane. [102] of 89 nm.

It is noted that the value of the d ₅ o (laser) and the size of the coherent domain have the same order of magnitude, which confirms the monocrystalline nature of the particles. The product obtained emits in blue (450 nm) and green (515 nm) under excitation at 254 nm. Figure 6 is the emission spectrum of this suspension.

Claims

Barium and magnesium aluminate, characterized in that it is in the form of a suspension in a liquid phase of substantially monocrystalline particles of average size between 80 nm and 400 nm.

2- Aluminate according to claim 1, characterized in that the particles have a mean size of between 80 nm and 200 nm, more particularly between 100 nm and 200 nm.

3- Aluminate according to one of the preceding claims, characterized in that the particles have a dispersion index of at most 0.7.

4. Aluminate according to one of the preceding claims, characterized in that it is crystallized in the form of a pure phase.

5- Aluminate according to one of the preceding claims, characterized in that it corresponds to the formula (I) a (Bai-dM ¹ dO) .b (Mgi- _e M ² eO). C (AI ₂ Os) in which:

M ¹ denotes a rare earth which may be more particularly gadolinium, terbium, yttrium, ytterbium, europium, neodymium and dysprosium; M ² denotes zinc, manganese or cobalt; a, b, c, d and e satisfy the relationships: 0.25 ≤ a ≤ 2; 0 <b ≤ 2; 3 ≤ c ≤ 9; 0 ≤ d ≤ 0.4 and 0 ≤ e ≤ 0.6.

6. Aluminate according to claim 5, characterized in that it corresponds to the above formula (I) wherein a = b = 1 and c = 5; where a = b = 1 and c = 7 or a = 1; b = 2 and c = 8.

7- Aluminate characterized in that it is in the form of a powder capable of giving, after redispersion in a liquid phase, the aluminate in suspension form according to one of the preceding claims.

8- A method for preparing an aluminate according to one of claims 1 to 6, characterized in that it comprises the following steps: - a liquid mixture is formed comprising compounds of aluminum and other elements falling within the aluminate composition; said mixture is spray-dried;

the dried product is calcined;

a wet grinding of the product resulting from the calcination is carried out.

9- Method according to claim 8, characterized in that the compound of aluminum used a soil of this element.

10- Process according to claim 8 or 9, characterized in that nitrates are used as compounds of aluminum and the aforementioned elements.

11 - Luminescent device, characterized in that it comprises, or in that it is manufactured using an aluminate according to one of claims 1 to 7 or an aluminate obtained by the process according to one of claims 8 to 10 .

12- plasma system, characterized in that it comprises, or in that it is manufactured using an aluminate according to one of claims 1 to 7 or an aluminate obtained by the process according to one of claims 8 to 10.

13- mercury vapor lamp, characterized in that it comprises, or in that it is manufactured using an aluminate according to one of claims 1 to 7 or an aluminate obtained by the method according to one of claims 8 to 10.

14- luminescent material, characterized in that it comprises, or in that it is manufactured using an aluminate according to one of claims 1 to 7 or an aluminate obtained by the method according to one of claims 8 to 10 .

15- The material of claim 14, characterized in that it is transparent and in that the aluminate above has a mean size of between 100 nm and 200 nm.

16. Luminescent system, characterized in that it comprises a material according to claim 14 or 15 and furthermore an excitation source.